The invention relates to a yarn withdrawal apparatus, as well as a texturing machine with the yarn withdrawal apparatus of the invention. Yarn withdrawal apparatus of this type are disclosed in WO 00/21866 A2 and corresponding U.S. Patent Publ. No. 2001/0037545.
To ensure a continuous operation in a yarn treating process, wherein a yarn is unwound from a feed package and further processed, the trailing yarn end of the feed yarn package may be joined to the leading yarn end of a reserve yarn package. In this connection, the joining occurs such that after having completely unwound the yarn from the feed yarn package, the yarn continues to unwind with no trouble from the reserve yarn package.
Normally, the joining occurs by knotting or splicing. A thick place that forms in the yarn as a result thereof, represents an unavoidable decrease in quality in the subsequent process. For this reason, the above cited documents propose to detect the transition from the feed yarn package to the reserve yarn package with the use of a sensor and to respond thereto in the treatment process. The sensor is provided between the packages, and it includes a movable yarn guide, which is caused to move by the yarn change between the packages, and whose position is detected.
A problem that arises in the above process is that the rapidly advancing yarn accelerates the movable yarn guide from an inactive position to a signaling position in a very short period of time due to the very high speed of the yarn. In so doing, it is possible that in the signaling position the yarn guide rebounds on its stop, and drops back to its inactive position. While it is possible to detect and electronically store this short stay, it is not desired from the viewpoint of faster and easier operability and operational safety in the event of electrical breakdowns that the yarn guide be in its released state in the inactive position.
Obvious attempts of solving the problem, such as, for example, the use of softly absorbent stop materials, have not produced satisfactory results. This also applies to bulk material fillings, which are integrated into the yarn guide, or other additional masses that are applied for the purpose of damping its movement. The reason is to be found in that because of the small mass of the yarn guide, in combination with the high yarn speed, a slight, undamped residual energy will suffice to rebound the yarn guide into its inactive position.
Likewise, other obvious solutions, such as bi-stable layers with the aid of permanent magnets, have not brought satisfactory results. Also, air damping and electromagnetically operating dampers are unsuited to bring about the required high damping forces. In this instance, one may consider as an additional problem the fact that the speed at which the yarn guide moves to the signaling position, varies very greatly. For this reason, it is hardly possible to adjust, for example, a friction brake such that it is equally reliable in operation both at high and at low speeds of the yarn guide.
It is therefore an object of the invention to provide a sensor for detecting and signaling the transfer of the withdrawn yarn between packages, and which has a movable yarn guide which reliably prevents the yarn guide from rebounding even at varying speeds.